The very high spatial resolution together with the spectral and contrast requirements of the EPICS instrument for the E-ELT telescope lead to the investigations of a Polarimetric Integral Field Unit (IFU) as a very promising concept. Since such an instrument concept had never been built before, the goal of this ESFRI project is to develop and demonstrate the technology of a polarimetric IFU.

Approach

The approach is to demonstrate the polarimetric IFU concept as an extension of the existing ExPo imaging polarimeter. The project has been split in different workpackages:

The EPICS instrument for the E-ELT telescope has been proposed not only to detect exoplanets that are smaller and closer to their host stars than is possible with the current generation of telescopes and instruments, but also to allow characterization of these planets. This calls for an instrument that can image exoplanets at very high spatial resolution and contrast and simultaneously provide spectral information on the observed planet(s).

With this in mind, a Polarimetric Integral Field Unit (IFU) was identified as a very promising concept during the EPICS Phase-A study. As such an instrument concept had never been built before, the goal of this ESFRI project is to develop and demonstrate the technology of a polarimetric IFU.

Approach

The approach is to demonstrate the polarimetric IFU concept as an extension of the existing ExPo imaging polarimeter. The project has been split in three workpackages:

Establish the top-level science requirements and goals, investigate –as broadly as possible- the technology and associated design options available and rate the different options on their ability to meet the requirements as well as availability/maturity.

WP2: Modelling of two competing approaches

Perform a first design iteration on the two most promising concepts. Model the resulting performance of the two designs and select the best performing for the next stage.

WP3: Prototyping of Selected Design

Perform a detailed design and construct the demonstration prototype. Characterize the performance of this prototype in the lab and develop the data-reduction pipeline. Compare lab performance with predicted performance and apply lessons learned to the EPICS polarimetric IFU concept.

Status

A postdoc has been working on the project since 1Q 2011.

In the initial phase of the project, microlens (ML)-based IFU concepts were identified as the most suitable: There is considerable experience with non-polarimetric ML-based IFU’s. Also, sampling an image plane with a microlens array in principal does not affect the polarization.Two ML-concepts were explored further and modelled: A single microlens sampling and a double-microlens, similar tot the SPHERE IFS concept (BIGRE). The single ML concept has been selected for prototyping.Two new technologies were also identified and adopted for the IFU design: The polarization grating (PG) for simultaneous polarizations splitting and wavelength dispersion, and polychromatic modulation for broad-band modulation of the polarization.During the design, a trade-off between spectral and spatial resolution was performed (Figure 1). Various ways of packing the double spectra were investigated and an optimal packing found.

Figure 1: The trade-off between spectral and spatial resolution, and its extremes.

The resulting prototype design has the following specifications:

Field of View: 10” x 10”

Wavelength range: 500 – 900 nm

Microlens sampling: 0.10”

Spectrum length: 70 pixels

Spectral resolution: R = 43

The prototype has been constructed and lab testing is well underway. A sample lab test result is presented in figure 2.

The data reduction pipeline is nearing completion and testing of the polychromatic modulation is currently (September 2013) in progress. The IFU prototype is scheduled for on-sky observations at the William Herschel Telescope on La Palma in November 2013.

Possible spin-off

In a separate context, an instrument concept for continuous spectro-polarimetric observations of Earthshine from the Moon has been developed and demonstrated in the lab. The design of this instrument, LOUPE, incorporates a number of elements of the polarimetric IFU design.

Applications of polarimetric IFU technology in areas such as biophysics and agriculture are potential spin-offs. Similar technologies are currently being used for instance for ophthalmology and plant-health monitoring..